[William Iorg]

In general terms, leading is caused by poor bullet fit in the throat and the bore. Leading occrus when gas excapes past the bullet as it travels up the bore.
Lyman is realesing the latest addition of the Cast Bullet Handbook and this is a good place to to start to understand leading and lubricant.
I believe Marshal covers leading in his load book.
If you have proper bullet fit you can shoot unlubricated bullets without leading issues. Norman Johnson is well known for this.